This application relates to a method of blanketing a container of liquid with an inert gas. In a particular aspect this application relates to a method for displacing atmospheric oxygen with an inert gas from the head-space of a container of a liquid.
Many liquids are reactive with oxygen. Such liquids include solutions of salts of anions in a reduced state, solutions of cationic salts and acids in a reduced state, ethers, highly flammable liquids the vapors of which form explosive mixtures with air over a broad range of concentrations, and liquids with low ignition temperatures.
One such liquid is nitromethane. It is known that, when nitromethane is confined in heavy-walled containers, it can be detonated by severe shock. It is also known that when the head space of the container is filled with nitrogen instead of air a significantly stronger shock is required for detonation. It is believed that the following sequence of events occurs in the presence of air: the vapors of nitromethane mixed with air are compressed by the shock under near-adiabatic conditions, thus raising the temperature of the vapor mixture to the ignition point where it burns, thereby increasing the pressure and temperature whereupon the liquid nitromethane begins to burn as a monopropellant and as the pressure and temperature rise, the entire body of the liquid detonates. In the absence of oxygen, there is no flammable vapor mixture to ignite and, for detonation to occur, the shock must be severe enough to raise the temperature of the vapors and/or liquid nitromethane to self-ignition temperatures. Hence, displacing air from such a container with an inert gas such as nitrogen, constitutes a safety measure.
The pressures required to achieve detonation are far greater than can be withstood by the usual shipping container, i.e. a Department of Transportation specification 17E drum. Repeated attempts have been made to detonate nitromethane in 17E drums using a variety of methods of delivering severe shock, but all that occurred was mechanical failure of the drum. It, therefore, seems that shipment of nitromethane in such containers, even with air in the head space, is without hazard due to shock. Nevertheless, the added margin of safety provided by displacing the oxygen, i.e. to less than 1% by vol., in the head space is deemed worthwhile and such a practice has been followed for a number of years.
Previously, the method employed was to fill the drum to overflowing thereby eliminating all head space, then using nitrogen under pressure, to displace sufficient liquid to provide the desired head space. This procedure apparently gave good results with little difficulty when the oxygen was determined soon after filling. However after only 24 hours of storage, it was often found that the oxygen content had risen well above 1%. In fact concentrations with 8-10% were common. Even after reblanketing with nitrogen for 2 minutes, as much as 7% or more oxygen could be determined after 24 hours. The source of the oxygen was traced to oxygen dissolved in the nitromethane in the distillation columns and in the check tank prior to delivery to storage.
Accordingly, an improved method is needed for effecting oxygen removal and nitrogen blanketing of containers filled with nitromethane or other liquids to be protected from oxygen.